The etching of smooth sidewalls in III-V compounds is important for optical applications. Scattering loss in electro-optical devices is proportional to sidewall roughness. Hence, the performance of devices such as waveguides, microdisc resonators, photonic crystal waveguides and photonic crystal resonators depends on reduction of the sidewall roughness. Single mode ridge waveguides in InP and GaAs typically require dimensions on the order of 0.5 μm to maintain single mode performance as scattering losses from the waveguide surface are a large component of the propagation loss. Most work on etching III-V compounds such as InP for low loss waveguides has focused on CH4:H2 chemistry in standard reactive ion etch (RIE) systems. A feature of sidewalls produced in standard RIE systems is that the sidewalls are sloped. Some electro-optical devices require highly vertical sidewall geometries for improved device performance. For example, in photonic crystal lattices, it is important to provide highly vertical sidewall geometries to enable large photonic bandgaps for device performance.
Hence, it is desirable to have etch chemistries that enable highly vertical sidewall geometries with smooth sidewalls.
Inductively coupled plasma (ICP) etch systems typically produce a higher degree of vertical etches for most materials due to the increase in density of active species. However the chemistry selection still plays an important role in obtaining high aspect ratio etching. Typically, a CH4:H2 based chemistry is used for etching InP; however, CH4:H2 based chemistry has difficulty etching very high apect ratios. Using chlorine based chemistry is problematic for obtaining high aspect ratio etching due to the highly reactive nature of chlorine. Recent work by Mirkarimi (see Ser. No. 10/765,647) has shown the usefulness of using HBr:CH4:H2 chemistry to achieve deep etching in III-V compounds. However, the sidewalls of the etched III-V structure in some quaternary compositions such as InGaAsP exhibit rough sidewalls when using Hbr:CH4:H2 chemistry.